Liquid chiller evaporator

ABSTRACT

A shell and coil type heat exchanger evaporator provides cooling for industrial coolants characterized by high viscosity and a poor heat transfer coefficient. A large heat transfer surface area is provided within a limited volume, and without a high coolant pressure drop. A short initial cool down period is provided due to minimized heat exchanger mass and minimized volume of liquid coolant in the heat exchanger during initial cool down.

[0001] This application claims the benefit of earlier filed and pendingprovisional application No. 60/214,565.

BACKGROUND OF THE INVENTION

[0002] This invention is directed to providing industrial coolants,characterized by high viscosity and a poor heat transfer coefficient,flowing through an evaporator at a rate of 200 grams/sec (2 gallons/min)at temperatures between −80 and −100 C. More particularly this inventiondefines a highly efficient and compact cylindrical evaporatorconstruction intended to provide cooling to Galden HT 70 (a commercialcoolant) while maintaining a pressure drop across the evaporator notexceeding 2 PSI. The thermal masses of the evaporator and of the coolantin contact with the evaporator during the initial cool down of theevaporator are minimized.

[0003] Refrigeration systems have been in existence since the early1900s, when reliable sealed refrigeration systems were developed. Sincethat time, improvements in refrigeration technology have proven theirutility in both residential and industrial settings. In particular, verylow temperature refrigeration systems, colder than −20 C., currentlyprovide essential industrial functions in biomedical applications,cryoelectronics, coating operations conducted in a vacuum (i.e. physicalvapor deposition), semiconductor manufacturing applications, control ofchemical reactions and pharmaceutical manufacturing processes. Anotherapplication involves thermal radiation shielding. In this applicationlarge panels are cooled to very low temperatures. These cooled panelsintercept radiant heat from vacuum chamber surfaces and heaters. Thiscan reduce the heat load on surfaces being cooled to lower temperaturesthan the panels. Yet another application is the removal of heat fromobjects being manufactured. In some cases the object is an aluminum discfor a computer hard drive, a silicon wafer for an integrated circuit, orthe material for a flat panel display. In these cases the very lowtemperature provides a means for removing heat from these objects morerapidly than other means, even though the object's final temperature atthe end of the process step may be higher than room temperature.Further, some applications involving hard disc drive media, siliconwafers, or flat panel display material, include the deposition ofmaterial onto these objects. In such cases heat is released from theobject as a result of the deposition and this heat must be removed whilemaintaining the object within prescribed temperatures. Cooling asurface, like a platen, is the typical means of removing heat from suchobjects.

[0004] In many of these applications, such as the semiconductor devicemanufacturing industry, it is necessary that refrigeration systemsprovide very low temperature refrigeration to highly viscous industrialcoolants with poor heat transfer coefficients. Highly viscous coolantsprovide several challenges and limitations to such systems, especiallythe evaporator of the refrigeration system. Additionally, in many suchapplications evaporator designs are further limited by size restrictionsand a necessity to be compatible with customer systems already in place.A coolant is used as an intermediate fluid instead of direct thermalcontact with the refrigerant in cases where the process tubing and heatexchanger are not rated for high design pressures required byrefrigeration processes. Using a secondary coolant (usually a liquid,but sometimes a gas) allows the process at the load where the heat isremoved to be operated with the coolant at much lower pressure than thepressure that the refrigerant process must operate.

[0005] This invention relates to refrigeration systems which providerefrigeration at temperatures between −20 C. and −150 C. by use of asecondary cooling fluid or coolant. The temperatures encompassed in thisrange are variously referred to as low, ultra low and cryogenic. Forpurposes of this application the term “very low” or very low temperaturewill be used to mean the temperature range of −20 C. to −150 C.

[0006] Industrial applications that require very low temperature coolingoften find it necessary to provide such cooling to liquid coolants thatbecome highly viscous at such temperatures. As a liquid coolant ispumped through a closed loop system, the pressure drop experienced bythe coolant as it flows through the evaporator affects the heat load onthe refrigeration system, since higher coolant pressure drops requiregreater pump work. Greater pump work results in a greater increase inthe fluid temperature rise during the pumping process and results in ahigher heat load of the refrigeration system.

[0007] Many system configurations require limitations on the size ofcomponents, such as an evaporator. In the case of evaporator sizerestrictions, it is still necessary for the evaporator to supply therequired cooling, or it is of no use. Evaporators typically achieveimproved heat transfer effectiveness and overall refrigeration cycleefficiencies by including a larger heat transfer area. However, theinclusion of a large heat transfer surface area in a limited volume maypresent a significant challenge.

[0008] Many systems also require a quick initial cool down time. This iscomplicated by the high viscosity and low thermal conductivity of verylow temperature industrial coolants. These physical limitations tend toresult in larger heat exchangers since tight fin spacing increasespressure drop. Larger heat exchangers have more mass to be cooled oninitial cool down. In addition, a larger heat exchanger typicallyrequires a larger volume to be filled with liquid. This large volume ofliquid represents an additional mass to be cooled initially. Therefore,an effective design minimizes heat exchanger mass and coolant volume,while maximizing flow passages (fin spacing). Due to the very highviscosity of the fluid, heat exchangers often will be operating with thefluid flow in the laminar flow regime. To minimize pressure drop, verylow Reynolds number flow is required. A limitation of laminar flow isthat fully developed laminar flow is difficult to alter in a way thatenhances the heat transfer rate. Therefore, an effective, compact designmust prevent fully developed flow. Understanding of fully developed flowand the development of fully developed flow relates to the formation ofa boundary layer. The physics of boundary layers is well known to thoseskilled in the art of heat exchanger design. For reference, an excellentdiscussion of laminar flow heat transfer in boundary layers and fullydeveloped laminar flow is given by “Convective Heat and Mass Transfer,”Kays and Crawford, McGraw Hill, 1980.

[0009] Galden HT 70 is an industrial coolant widely used in thesemiconductor manufacturing industry characterized by high viscosity(especially at cold temperatures), a poor heat transfer coefficient, anda tendency to freeze out at temperatures below −120 C. Thesecharacteristics present many limitations and challenges to the design ofan evaporator that is to remove heat from such a coolant.

[0010] Similar limitations are also experienced by other liquid heattransfer coolants used to provide heat transfer at temperatures below−20 C. Although the current design was originally developed for use withGalden HT 70 it can also be used for other similarly high viscousliquids.

[0011] Such very low temperatures are needed for a variety of industrialapplications. In the semiconductor industry such very low temperaturesare important for processing of semiconductor wafers. In one suchexample, the deposition of material on a wafer causes heat to berejected to the wafer, which heat must be removed. Further, suchprocesses must take place within a specified temperature range.Frequently, the process design requires cooling temperatures of −20 C.or colder to achieve desired process conditions. Additionally, very lowtemperature cooling is needed when the completed wafers are tested.

BACKGROUND PATENTS

[0012] U.S. Pat. No. 5,704,123, “Method of making folded, bent andre-expanded heat exchanger tube and assemblies,” assigned to Peerless ofAmerica, Incorporated (Aptakisic, Ill.), describes a heat exchangerassembly of the side-entry type including at least one fin set and anelongated heat exchanger tube having a collapsed sidewall extendingsubstantially the length of the tube which permits the bending of theelongated heat exchanger tube at the return bend portions and permitsexpansion of the elongated tube to engage the fin set. Method andapparatus for making the elongated heat exchanger tube having thecollapsed sidewall substantially extending the length of the tube aswell as methods of making heat exchanger assemblies are disclosed.

[0013] U.S. Pat. No. 5,538,075, “Arcuate tubular evaporator heatexchanger,” assigned to Eubank Manufacturing Enterprises, Inc.(Longview, Tex.), describes an indoor heat exchange unit and method ofmaking same characterized by an arcuate coil shape heat exchange unitmade by bending a single tubing row, planar heat exchange unit to fitwithin a limited space with an open inlet at one end and blocked at theother end so as to force air to flow past the coil and transfer heatthrough the fins and tubes of the coil in the process. Also disclosedare preferred embodiments in which an air circulation fan circulates airand where a thermostat controls the flow of heat exchange fluid throughthe coil as the air is passed through the arcuate coil to obtain apredetermined temperature, or the like, in the air.

[0014] U.S. Pat. No. 4,766,736, “Evaporator coil heat exchangerassembly,” assigned to Thermal King Corporation (Minneapolis, Minn.),describes an evaporator coil heat exchanger assembly of a refrigerationsystem, such as a transport refrigeration system, which includes anelectrical heating element for rapid defrosting of the evaporator coilwith its attendant refrigerant carrying tubes and cooling fins. Theelectrical heating element is quickly attached to the collective edgesof the cooling fins by a plurality of low cost spring retainer clipswhich have one portion which extends between two closely spaced coolingfins to hook a refrigerant carrying tube, and another portion whichholds the heating element against edges of the cooling fins with aspring force. The spring retainer clips may be just as quickly removedshould the heating element require replacement. Each spring retainerclip is formed from a single piece of metallic wire having first andsecond curved end sections, and an intermediate portion which functionsboth as a spring and as a handle.

[0015] U.S. Pat. No. 4,175,617, “Skewed turn coiled tube heat exchangerfor refrigerator evaporators,” assigned to General Electric Company(Louisville, Ky.), describes a heat exchanger for refrigeratorevaporators of the type consisting of helically coiledrefrigerant-carrying tubing with radially inward extending fins formedalong the length of the coiled tubing, with the air to be refrigerateddirected across the axis of the coil turns. The coil turns are skewedfrom the helix angle to expose a greater proportion of the fins into theair flow path between the coil turns so as to increase the air flowcontact with the fins. The skewing is created by relatively offsettingopposite portions of the coil turns along the air flow path across thehelical coil to increase the obliqueness of a portion of each coil turnwith respect to the direction of air flow. The tube coil includessections folded into a side-by-side relationship, with the skewdirection of the coil sections placed in reverse orientations toincrease the length of the flow path of the air circulated through thecoils and to position gaps between each of the coil turns in eithersection opposite the areas in the other coil section occupied by radialfins.

[0016] U.S. Pat. No. 4,116,270, “Tubular coiled heat exchanger anddevice for manufacturing same,” invented by Marushkin, Zelenov, Kozlov,et al (all of Moscow), describes a tubular coiled heat exchanger adaptedfor cooling or heating of various fluids in various fields of industry.The heat exchanger comprises a shell and a core around which tubeshaving essentially the same length are wound-at least in two layers. Thetubes may have grooves projecting into the tubes to intensify heatabstraction within the tubes. A member of a streamlined cross-section,such as, a wire, adapted for forming fins is wound around the tubes witha pitch of at least twice the diameter of the wire. The tubes are woundaround the core so that the tops of the fins of each tube coil comealternately in contact with those of the fins and with tube surfaces ofadjacent coils. The above embodiment of the heat exchanger makes itpossible to vary the number of turns of the tubes when winding the tubesbeing set once without any distance pieces between the layers. Thisensures the manufacture of a highly compact heat exchanger featuringhigh thermal and hydrodynamic properties. The heat exchanger may proveto be most advantageous in cryogenics, in plants for liquefaction andseparation of natural gas in particular.

SUMMARY OF THE INVENTION

[0017] The present invention is a shell and coil type heat exchangerevaporator that provides desired cooling, for example, to Galden HT 70,an industrial coolant characterized by high viscosity and a poor heattransfer coefficient. The coolant flows through the evaporator at arate, for this example, of 200 grams/sec (2 gallons/min) at temperaturesbetween −80 and −100 C. with a maximum pressure drop across theevaporator of 2 PSI. The present invention in this example transfers 500W at −80 C. and provides a heat transfer surface area between 1.4 and1.7 square meters included within limited evaporator dimensions ofapproximately twelve inches in length and three to four inches indiameter.

[0018] One advantage of the use of the shell and coil type heatexchanging evaporator in accordance with the invention is that itachieves the desired cooling, for example, of Galden HT 70, by providinga large heat transfer surface area within a limited volume, and withouta high coolant pressure drop.

[0019] A second advantage of the use of the shell and coil type heatexchanging evaporator in accordance with the invention is that it allowsa short initial cool down period due to the minimized heat exchangermass and the minimized volume of liquid coolant to be cooled duringinitial cool down.

[0020] A third advantage is that the current invention describes arefrigerant mixture that can be used to remove heat from the coolantwhen used in a refrigeration process which cools the coolant heatexchanger (refrigerant evaporator).

[0021] A fourth advantage is that the current invention can also be usedto cool a gas stream without a high gas pressure drop.

[0022] Still other objects and advantages of the invention will beapparent in the specification. The invention accordingly comprises thefeatures of construction, combinations of elements, and arrangements ofparts, which will be exemplified in the constructions hereinafter setforth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For better understanding of the invention, reference is had tothe following description taken in connection with the accompanyingdrawings, in which:

[0024]FIG. 1 is a schematic of a very low temperature refrigerationsystem with a shell and coil type heat exchanging evaporator inaccordance with the invention; and

[0025]FIG. 2 is a schematic of the shell and coil type heat exchangingevaporator of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026]FIG. 1 is a block diagram of one generic construction using theshell and coil type heat exchanging evaporator in accordance with theinvention. FIG. 1 shows a conventional refrigeration system 100 thatincludes a refrigeration unit 102 feeding an evaporator 104 via arefrigerant supply line 106. The refrigerant supply line 106 feeds aflow metering device (FMD) 108 which in turn feeds evaporator 104. Theloop is closed from evaporator 104 back to refrigeration unit 102 via arefrigerant return line 110. Coolant is pumped into evaporator 104 froma secondary coolant loop (not shown) via a coolant inlet 114, andcoolant is pumped out of evaporator 104 into the secondary coolant loopvia a coolant outlet 116. Furthermore, a liquid drain valve 118 isconnected to coolant inlet 114.

[0027] Refrigeration system 100 is a conventional refrigeration systemand its basic operation, which is the removal and relocation of heat, iswell known in the art. All components within refrigeration system 100are connected for refrigerant flow.

[0028] Refrigeration unit 102 is any refrigeration system or process,such as a single-refrigerant system, a mixed-refrigerant system, normalrefrigeration processes, an individual stage of a cascade refrigerationprocesses, an auto-refrigerating cascade cycle, or a Kleemenko cycle.

[0029] More specifically, refrigeration unit 102 may be the IGC PolycoldSystems Inc. (San Rafael, Calif.) system (i.e., autorefrigeratingcascade process), IGC APD Cryogenics (Allentown, Pa.) system with singleexpansion device (i.e., single stage cryocooler having no phaseseparation, Longsworth U.S. Pat. No. 5,441,658), Missimer type cycle(i.e., autorefrigerating cascade, Missimer U.S. Pat. No. 3,768,273),Kleemenko type (i.e., two phase separator system), or a single phaseseparator system. Also, refrigeration unit 102 may be variations onthese processes such as described in Forrest U.S. Pat. No. 4,597,267 andMissimer U.S. Pat. No. 4,535,597.

[0030] Several additional basic variations of refrigeration unit 102shown in FIG. 1 are possible. Refrigeration unit 102 may be one stage ofa cascaded system, wherein the condensation of refrigerant is caused byvery low temperature refrigerant from another stage of refrigeration.Similarly, the refrigerant produced by the refrigeration unit 102 may beused to cool and liquefy refrigerant of a lower temperature cascadeprocess. Further, the refrigeration unit shown in FIG. 1 uses at least asingle compressor. It is recognized that the refrigerant vaporcompression effect can be obtained using two or more compressors inparallel, or that the compression process may be broken up into stagesvia compressors in series or a two stage or multi stage compressor. Allof these possible variations are considered to be within the scope ofthis disclosure.

[0031] FMD 108 is any conventional flow metering device, such as acapillary tube, an orifice, a proportional valve with control feedback,or any restrictive element that controls flow. FMD 108 regulates thecorrect amount of refrigerant flowing into evaporator 104, much like athrottle. FMD 108 is one of the elements separating high-pressure andlow-pressure regions within refrigeration system 100.

[0032] Liquid drain valve 118 is a normally closed valve that is locatedat the bottom of the secondary coolant loop. When necessary, liquidcoolant is drained out of refrigeration system 100 via liquid drainvalve 118. The ability to recover the liquid coolant is important due tothe high cost of liquid coolants. A further function of liquid drainvalve 118 is to provide a port that is used to fill the coolant passagesof refrigeration system 100. This is also very important because trappedair or gas in a liquid loop decreases pump displacement. Therefore,liquid drain valve 118 must be the lowest point of the secondary coolantloop and must have direct plumbing that is continuously downward toprevent the possibility of having any trapped air or gas.

[0033] Evaporator 104 of the present invention is a unique shell andfinned coil heat exchanger. Refrigerant evaporates and absorbs heat fromthe coolant within evaporator 104. Evaporator 104 is described in detailin FIG. 2.

[0034]FIG. 2 is a detail drawing of a first embodiment of evaporator104, in accordance with the invention. Evaporator 104 includes a coolantinlet 202/114, a coolant outlet 204/116, a refrigerant inlet 206, arefrigerant outlet 208, an outer cylinder 210, an inner cylinder 212,coiled tubing 214, an outer cap 216, an inner cap 218, and fins 220.

[0035] Coolant inlet 202 is a segment of a coolant line that provides acoolant flow path to the inside of evaporator 104 at cap 216 throughwhich coolant enters evaporator 104. Conversely, coolant outlet 204 is asegment of the coolant line that provides a coolant flow path with theinside of evaporator 104 at cap 218, through which coolant exitsevaporator 104.

[0036] Refrigerant inlet 206 is a segment of a refrigerant line whichconnects to coiled tubing 214 through which refrigerant entersevaporator 104. Conversely, refrigerant outlet 208 is fed by refrigerantleaving coiled tubing 214.

[0037] Cylinder 210 is the outer shell of evaporator 104. Cylinder 212is a sealed hollow cylinder. The cylinder 212 restricts the flow ofcoolant through evaporator 104 to the volume defined by the volume ofcylinder 210 minus the volume of cylinder 212, thereby forcing coolantto flow across the heat transfer surface area provided by fins 220, andeliminating potential coolant bypass around the fins.

[0038] Coiled tubing 214 is a helically wound length of ⅜ inch OD coppertubing along which fins 220 are attached. Each turn of coil of finnedtubing 214 comes into contact with the next turn at the fin tips. Forthe specific arrangement tested, the tubing diameter was 0.375″ and hada fin height of 0.25″ resulting in an overall diameter of 0.875 inch.This results in a pitch of about 0.875″. Coiled tubing 214 iscommercially available from a supplier (such as Heat Exchange AppliedTechnology, Orrville, Ohio) in a straight length or pre-coiled andincludes fins 220 attached at the desired fin spacing and fin lengths.Typical fin spacing is between 0.06 and 0.08 inch. Typical fin height isbetween 0.25 and 0.38 inch.

[0039] Fins 220 are made of copper and have a thickness of about 0.015inch and extend 0.25 inch from the tubing. The fins are typically madeof a continuous metal ribbon which is then wrapped onto the tube withthe width of the ribbon oriented perpendicular to the tube length. Theresulting assembly has a diameter equal to the tube diameter plus twofin heights. The fins are wrapped in a helix so that after completingone wrap around the tube, the fins are offset by one fin spacing. Fins220 are fixed to the tube by a tension wind method or by brazing to thecoiled tubing 214 (prior to coiling of the tube), extending from thesurface of coiled tubing 214 from the inner surface of cylinder 212 tothe inner surface of cylinder 210. The goal of the attachment method isto provide adequate mechanical stability to allow coiling of the fin andtube assembly, and to provide effective thermal conduction from the finsto the tubing. Typically the fins and tubing are made of copper.Alternatively, the material of the tubing and the fins may be ofaluminum or stainless steel.

[0040] Coiled tubing 214 is formed using a small mandrill and a rotatingmachine by holding stationary one end of the straight length of tubingand forming a coil. Such methods of forming coiled tubing 214 lengthsare well known to the industry.

[0041] Cap 216 seals one end of the evaporator 104. The opposite end ofcylinder 210 is also sealed by a cap 224. Likewise, inner cylinder 212is sealed by two caps, one of which is cap 218, the other is not shownfor clarity in the drawing.

[0042] Support 222 connects to the inner cylinder 212 and to the cap 216to support the inner cylinder within the outer cylinder 210.

[0043] The material, and material thickness of the cylinders 210 and212, and of the caps 216, 224 and 218 are selected to provide the properpressure rating needed for the coolant loop when operating at very lowtemperatures. In operation the coolant mat be pressurized up to 100psig. This pressure is internal to cylinder 210 and caps 216 andexternal to cylinder 212, caps 218, and the coiled tubing 214. Sincethermal mass must be minimized, pressurization has an impact on thematerial selected. Additionally the method used to join the caps to thecylinder affects the material selection. For example, use of a weldingprocess will typically require aluminum or stainless steel whereas usedof a brazing process will typically require copper or stainless steel.The shape of the caps 216, 224, 218 are selected to provide an adequatepressure rating.

[0044]FIG. 2 shows a vertical orientation for the evaporator. This isimportant for purposes of enabling removal of coolant, or filling ofcoolant. For this purpose, the liquid drain valve 118 of FIG. 1 needs tobe connected to the line connecting the coolant inlet 202 to the rest ofthe coolant loop.

[0045] Evaporator 104 was designed specifically for providingrefrigeration to the industrial coolant Galden HT 70. Galden HT 70 isused in applications such as the semiconductor manufacturing industryfor cooling the chucks upon which silicon wafers are retained duringetching processes, and is characterized by a high viscosity and a poorheat transfer coefficient.

[0046] The special coolant to which refrigeration was to be supplied,combined with customer restrictions, presented several limitations tothe design of evaporator 104. Firstly, the coolant is cooled to −80 C.with 500 W of refrigeration capacity. Further, the flow rate of thecoolant is fixed at about 200 grams/sec (2 gallons/min), by a coldtemperature pump and the circulation system supplied by the customer.Additionally, at these operating temperatures the viscosity of thecoolant is very high, about 10 centistokes at −80 C. A large pressuredrop across the evaporator is undesirable because it affects the pumpenergy input and the heat load on the refrigeration system. Morespecifically, higher coolant pressure drops require higher input power,and higher input power to achieve a given amount of coolant heat removalresults in lower efficiency. Evaporator 104 was limited to a 2 PSIpressure drop across it. Further, the overall size of evaporator 104 wasrestricted due to spatial constraints of the system.

[0047] In operation, evaporator 104 is a cylindrical counter-flow shelland coil heat exchanger whose dimensions are about twelve inches inlength and between three and four inches in diameter. The evaporatorprovides refrigeration to the coolant as follows. Warm liquid coolantenters evaporator 104 at coolant inlet 202 and is distributed evenly byvirtue of the symmetrical arrangement of the components. Coolant fillsthe volume of evaporator 104 between cylinder 212 and cylinder 210 andis pumped toward coolant outlet 204 at a pressure between 10 and 20 PSIat a flow rate of 200 grams/sec (2 gallons/min). Refrigerant, meanwhile,is flowing through evaporator 104 in the opposite direction, fromrefrigerant inlet 206 through coiled tubing 214 and out of evaporator104 at refrigerant outlet 208. The refrigerant flowing through theevaporator has a sufficient bulk heat transfer such that its temperaturerises no more than three times the temperature change of the coolant ascoolant is cooled by the evaporator. Use of a mixed refrigerant ispreferred since it provides a significant temperature difference betweenthe evaporator inlet 206 and outlet 208 and enables higher thermodynamicefficiency of the heat transfer since a more constant temperaturedifference is provided between refrigerant and coolant.

[0048] Coolant and refrigerant come into thermal contact while flowingcounter to one another within evaporator 104 and heat is extracted fromthe coolant by the refrigerant and heats the refrigerant, therebycooling the coolant.

[0049] The heat transfer that takes place within evaporator 104 islimited by the Galden, whose heat transfer coefficient is between 30 and40 W/m²−K. Although the heat transfer coefficient of the refrigerant isbetween 600 and 800 W/m²−K, the overall heat transfer coefficient isrestricted to the lesser of the two values, and thus is between 30 and40 W/m²−K. This fact affects the heat exchange, which is defined by

Q=k FΔT

[0050] where Q is the heat exchanged; k is the overall heat transfercoefficient (between 30 and 40 W/m²−K, as stated above); F is the heattransfer surface area; and ΔT is between 3 and 4 degrees Celsius(dictated by the customer).

[0051] The only parameter of the above equation that is not fixed in aparticular construction is F, the heat transfer surface area. Fins 220provide a large surface area for heat transfer between the coolant andthe refrigerant within the restricted volume of evaporator 104, therebyproviding the desired cooling. About fourteen fins 220 were attached, inthe above example, along every inch of coiled tubing 214, providing aheat transfer area between 1.4 and 1.7 square meters.

[0052] This design operates with a Reynolds number of about 10, based onthe spacing between the fins. Having subsequent rows of finned tubingwhose fins are offset from the prior row is preferred because itenhances heat transfer by preventing a fully developed hydrodynamic andthermal boundary layer. Specifically, it is preferred that fluid flowingnear the in the center (maximum space between the fluid and the fins) oftwo fins when passing through fins attached to one coil, will flow muchcloser to the fins (minimum spacing between the fluid and a fin)attached to the next coil that the fluid passes by. Although this willcreate more pressure drop than if the fins of each coil are aligned, itimproves heat transfer which enables a reduction in heat exchanger massand coolant volume in the heat exchanger.

[0053] In a second embodiment, in accordance with the invention, theevaporator described in the first embodiment is combined with a mixedrefrigerant including individual refrigerant components with boilingpoints that vary by at least 90 C. from the coldest boiling component tothe warmest boiling component in the mixture.

[0054] A list of refrigerants that can be mixed together to provide therequired refrigeration performance are listed in Table 1. TABLE 1Refrigerant composition Range (% by Ingredient Name weight) Example (%by weight) Argon or 0-20% Nitrogen R-14 10-60%  22% R-14 At least one of5-40% 9% R-23, R-23 or ethane At least one of 5-30% 9% R-125, or R-R-125 143a, or R-32, or R- 134a, or R- 227ea, or R-218, or R-152a Atleast one of 0-70% 60% R-236fa, or R- R-236fa 245fa, or R- 236ea or R-245ca, or E-347, or R-4112, or R-4310meec

[0055] Table 1 is a listing of the refrigerant mixture used inconjunction with the present invention, including Argon or Nitrogen,along with refrigerants R-14, R-23, R-125, R-32, R-134a, R-227ea, R-218,R-152a, R-236fa, R-245fa, R-245ca, R-236ea. With the exception of E-347,R-4112, and R-4310meec, all refrigerants listed are designated inaccordance with American Society of Heating and Refrigeration and AirConditioning Engineering (ASHRAE) standard number 34.

[0056] E-347 is also known as1-(methoxy)-1,1,2,2,3,3,3-heptafluoropropane (also CH3-O-CF2-CF2-CF3),3M product reference Hydrofluoroether 301.

[0057] R-4112 is known as dodecafluoropentane (also CF3CF2CF2CF2CF3).

[0058] R-4310meec (1,1,1,2,2,3,4,5,5,5-decafluoropentane) iscommercialized as a solvent by DuPont and is known by the trade nameVertrel XF.

[0059] For most applications it is desired to have refrigerantcomponents that are nonflammable and nontoxic. The specific blend listedmeets these criteria and are the preferred refrigerants to use. Thisblend was applied in a refrigeration system without any phaseseparation, as described by the Longsworth Patent (cited above).

[0060] The other refrigerants listed are alternative refrigerants.Refrigerant R-245fa is a refrigerant for use with or instead of R-236fa.Likewise, R-236ea and R-245ca are good candidates for use with orinstead of R-236fa should they be available.

[0061] Refrigerant E-347 is also another good refrigerant for use withor instead of R-236fa. However, a permissible exposure limit for E-347is not yet established. Once known, its exposure limits will need to bereviewed for customer acceptance.

[0062] The following refrigerants are known to be flammable which makesthem less desirable for use: R-32, R-143a, R-152a, ethane.

[0063] Refrigerants R-227ea and R-134a are not optimal refrigerants formost very low temperature applications because their boiling point isnot optimum. Instead, R-125 is preferred.

[0064] Refrigerants R-218 and R-4112 are fully fluorinated compounds andhave high global warming potentials. Therefore they are also notpreferred. Additionally they are expected to produce a lowerrefrigeration system efficiency than the preferred refrigerants.

[0065] R-4310meec is considered to be toxic which makes it a lessdesirable refrigerant than the preferred refrigerants.

[0066] Table 1 lists the ingredients of the refrigerant mixture usedfavorably in conjunction with the present invention. The refrigerantmixture of Table 1 is characterized by a heat transfer coefficientbetween 600 and 800 W/m2. Further, the specific example refrigerantmixture of Table 1 is a nonflammable, nonchlorinated refrigerant blend,desirable due to increasingly restrictive environmental regulations.

[0067] In a third embodiment the heat exchanger described in FIG. 2 isused with the refrigerant mixture described in the second embodiment tocool a gas while causing a low pressure drop in the gas being cooled bythe evaporator.

[0068] Some industrial applications make use of a gas as the coolant inwhich a blower is used to recirculate the gas within a cooling loop. Inthis case, as with the pumping of a liquid, excessive pressure dropacross the evaporator requires additional pumping work to be performedon the gas by the blower which ultimately increases the thermal load onthe evaporator. Therefore low pressure drop is essential for asuccessful design. Further, gasses typically have poor heat transferproperties not unlike the poor heat transfer of coolants like Galden HT70. Use of the heat exchanger described in the first embodiment enableseffective cooling of a gas stream with a low gas pressure drop.

[0069] In summary, a first feature of the invention is a means toachieve the desired cooling to Galden HT 70, an industrial coolantcharacterized by high viscosity and a poor heat transfer coefficient, byproviding a large heat transfer surface area within a limited volume.This is effected without a high coolant pressure drop. Specifically, theshell and coil type heat exchanging evaporator in accordance with theinvention provides the desired cooling to Galden HT 70 flowing throughthe evaporator at a rate of 200 g/s (2 gallons/min) at temperaturesbetween −80 and −100 C. with a maximum pressure drop across theevaporator of 2 PSI. The present invention allows the transfer of 500 Wat −80 C. and provides a heat transfer surface area between 1.4 and 1.7square meters included within the limited evaporator dimensions ofapproximately twelve inches in length and between three and four inchesin diameter.

[0070] A second feature of the invention is a means of allowing a shortinitial cool down period due to the minimized heat exchanger mass andthe minimized volume of liquid coolant to be cooled during initial cooldown. In speaking of “minimized”, this is a practical engineeringreduction in volume and mass and should not be construed as amathematical or scientific minimum derived with precise multi-decimaltolerances.

[0071] A third feature of the invention is the use of a mixedrefrigerant system with the evaporator design described where the mixedrefrigerant is comprised of two refrigerants whose boiling points differby at least 90 C.

In the claims:
 1. A liquid chiller for cooling low temperature coolant,said coolant in steady state operation being cooled from T2 to T1 at apredetermined mass flow and pressure drop, comprising: a cylindricalouter casing having an initial internal volume, and including a coolantinlet and a coolant outlet, said coolant inlet and outlet communicatingwith said initial internal volume, said outer casing further including arefrigerant inlet and refrigerant outlet; an inner cylinder within saidouter casing and having a common longitudinal axis with said casing,said inner cylinder occupying a substantial portion of said initialinternal volume; a finned tubing of extended length connected betweensaid refrigerant inlet and said refrigerant outlet for refrigerant flowtherethrough, said finned tubing being wrapped around and contactingsaid inner cylinder and further occupying said initial internal volume,fins of said tubing extending toward an inner surface of said outercasing; in operation, coolant entering said volume at said coolant inletflowing over said finned tubing to said coolant outlet, a coldrefrigerant flowing through said finned tubing from said refrigerantinlet to said refrigerant outlet absorbing heat from said coolant, saidouter casing and inner cylinder being constructed in materials anddimensions, and said fins and tubing being selected, to provide low massand a reduced portion of said initial volume, said reduced portion beingunoccupied by said inner cylinder and said finned tubing and fillablewith coolant, said chiller with said low mass and reduced portion ofinternal volume providing said T2,T1, mass flow, and pressure drop insteady state operation, and rapid cool down at start up.
 2. A liquidchiller as in claim 1, wherein said outer casing and said inner cylinderare circular cylinders.
 3. A liquid chiller as in claim 1, whereincloseness of said fins to said inner surface of said outer casingprevents any substantial bypass of said fins by said coolant flow duringoperation of said chiller.
 4. A liquid chiller as in claim 3, whereinsaid chiller is sized for use with a refrigerant that is one of a singlerefrigerant and a mixed refrigerant.
 5. A liquid chiller as in claim 3,wherein said coolant cab flow counterflow to said refrigerant for heattransfer.
 6. A liquid chiller as in claim 3, wherein said chiller issized for use with a coolant that is one of a liquid and a gas.
 7. Aliquid chiller as in claim 6, wherein said coolant is Galden HT
 70. 8. Aliquid chiller as in claim 4, wherein said refrigerant is a mixture ofat least two components having normal boiling points at least 90 degreesC. apart.
 9. A chiller as in claim 4, wherein said refrigerant is amixed refrigerant that is approximately 22% R-14, approximately 9% R-23,approximately 9% R-125, and approximately 60% R-236_(fa=l .)
 10. Aliquid chiller as in claim 5, wherein the temperature increase ofrefrigerant passing through said chiller in operation is equal to orless than three times the temperature change in the coolant flow.
 11. Aliquid chiller as in claim 7, wherein said coolant when flowing oversaid finned surface has a Reynolds number of approximately
 10. 12. Aliquid chiller as in claim 1, wherein said inner cylinder is at leastpartially hollow and sealed off from said initial internal volume.
 13. Aliquid chiller as in claim 3, wherein said inner cylinder is at leastpartially hollow and sealed off from said initial internal volume.
 14. Aliquid chiller as in claim 7, wherein said inner cylinder is at leastpartially hollow and sealed off from said initial internal volume.
 15. Aclosed cycle low temperature refrigeration system comprising: arefrigeration unit for delivering low temperature, low pressurerefrigerant at an outlet, and receiving a return flow of saidrefrigerant at a higher temperature; a liquid chiller for cooling lowtemperature coolant, said coolant in steady state operation being cooledfrom T2 to T1 at a predetermined mass flow and pressure drop, saidliquid chiller including: a cylindrical outer casing having an initialinternal volume, and including a coolant inlet and a coolant outlet,said coolant inlet and outlet communicating with said initial internalvolume, said outer casing further including a refrigerant inletreceiving said refrigerant from said refrigeration unit and arefrigerant outlet returning said refrigerant to said refrigerationunit; an inner cylinder within said outer casing and having a commonlongitudinal axis with said casing, said inner cylinder occupying asubstantial portion of said initial internal volume; a finned tubing ofextended length connected between said refrigerant inlet and saidrefrigerant outlet for refrigerant flow therethrough, said finned tubingbeing wrapped around and contacting said inner cylinder and furtheroccupying said initial internal volume, fins of said tubing extendingtoward an inner surface of said outer casing; in operation, coolantentering said volume at said coolant inlet flowing over said finnedtubing to said coolant outlet, said cold refrigerant flowing throughsaid finned tubing from said refrigerant inlet to said refrigerantoutlet absorbing heat from said coolant, said outer casing and innercylinder being constructed in materials and dimensions, and said finsand tubing being selected, to provide low mass and a reduced portion ofsaid initial volume, said reduced portion being unoccupied by said innercylinder and said finned tubing and filled with said coolant, saidchiller with said low mass and reduced portion of internal volumeproviding said T2,T1, mass flow, and pressure drop in steady stateoperation, and rapid cool down at start up.
 16. A refrigeration systemas in claim 15, wherein said outer casing and said inner cylinder arecircular cylinders.
 17. A refrigeration system as in claim 15, whereincloseness of said fins to said inner surface of said outer casingprevents any substantial bypass of said fins by said coolant flow duringoperation of said chiller.
 18. A refrigeration system as in claim 17,wherein said refrigerant is one of a single refrigerant and a mixedrefrigerant.
 19. A refrigeration system as in claim 17, wherein saidcoolant flows counterflow to said refrigerant for heat transfer.
 20. Arefrigeration system as in claim 17, wherein said coolant is one of aliquid and a gas.
 21. A refrigeration system as in claim 20, whereinsaid coolant is Galden HT
 70. 22. A refrigeration system as in claim 18,wherein said refrigerant is a mixture of at least two components havingcomponent boiling points at least 90 degrees C. apart.
 23. Arefrigeration system as in claim 18, wherein said refrigerant is a mixedrefrigerant that is approximately 22% of R-14, approximately 9% of R-23,approximately 9% of R-125, and approximately 60% of R-236_(fa).
 24. Arefrigeration system as in claim 19, wherein the temperature increase ofrefrigerant passing through said chiller in operation is equal to orless than three times the temperature change in the coolant flow.
 25. Arefrigeration system as in claim 21, wherein said coolant when flowingover said finned surface has a Reynolds number of approximately
 10. 26.A refrigeration system as in claim 15, wherein said inner cylinder is atleast partially hollow and sealed off from said initial internal volume.27. A refrigeration system as in claim 17, wherein said inner cylinderis at least partially hollow and sealed off from said initial internalvolume.
 28. A refrigeration system as in claim 21, wherein said innercylinder is at least partially hollow and sealed off from said initialinternal volume.